calculating gravity after additions

Well, the easiest way would have been to calculate gravity before and after additions of cherry juice. You find out how much gravity the cherry juice contributed and add it to the initial gravity.
Calculators might get you in the ball park but in my experience it will often be off by at least 1-2%abv. Gotmead's mead calculator takes a range of sugar sources, including cherries. But there is a difference between using whole fruit or juice, the type of fruit you use, and the ripeness of fruit. Not to mention if there is added sugar in the juice, of course!

I played around a bit with the Gotmead calculator myself http://gotmead.com/blog/the-mead-calculator/
Here is what I did:
I locked the volume at 5.5gal (4gal apple juice +1.1gal cherry juice +~0.4 gal honey) and the honey value at 12.5lbs. The gravity came out to be 1.081. It doesn't matter how you get (at least close to) 1.082, as long as you have the right target volume. In other words I have created an equivalent must prior to the cherry addition.
Then add the cherry juice as additional sugars 2. I chose montmorency cherries since the other option is sweet, and I input 1.1gal. The gravity went up from 1.081 to 1.090.
If I were to choose the sweet cherries the gravity would have went up to 1.097 instead.

If I were making this batch myself I'd probably estimate my total starting gravity to be around 1.090. This is unless I know the exact volume of must or if I tasted or have experience with the juice and have reason to believe it has a different gravity

You are correct when you say that adding a lower gravity addition to a higher gravity must, your overall gravity should decrease. This will evidence itself in the math, when you factor in the volumes. Example:

If you had a 4 gallon must with a gravity of 1.080, and you add 1 gallon of, say, apple juice at 1.050. So you have 4 gallons at 80, and 1 gallon at 50. Total gravity points are 320 + 50 = 370, divided by 5 gallons, equals 74, or SG 1.074.

But in your situation you had a must that had already partially fermented, and was undoubtedly lower than your cherry juice addition, but you didn't state what the gravity was when you added the juice. Let's assume it was 1.020. So you had 4 gallons at 20 points, for 80 points. Then you added 1.1 gallons at 58 points, for 63.8. 80+63.8=143.8. Divided by the new total volume of 5.1 gallons = 1.028, an increase of 8 points.

Hope this makes sense, and has been helpful.

needitcheaper said:

i acknowledge that i am no expert, so i am not bashing someone who has the experience when i dont, but the math does not make sense to me on an intuitive level. if my must has an og of 1.082, and the cherry juice has an og of 1.058, then doesnt the total gravity have to go down, rather than increasing to 1.09?

Click to expand...

What you are saying is correct.
I left out a step which adjusts volume...

1. Simulate your gravity as it was in the beginning
Honey = 10.1lbs
Target gravity = 4.4gal (4gal cider, ~0.4galhoney)
The resulting gravity is 1.082 like you had in the beginning

2. Adjust volume to 5.5 from cherry juice (+1.1gal cherry juice)

3. Input additional sugars for cherry juice
Type Montmorency
Quantity 1.1 gal

Your Original gravity had you done all this at the start together would have been around 1.074
This is a rough estimate

You can also use the blending calculator:
SG1 = 1.082
Volume1 = 4.4
SG2 = 1.042 (Cherry juice sg for 1.1gal montmorency found through the mead calculator)
Volume2 = 1.1
Click on Blended SG = 1.074

Since my answers are close to yours I think this is what you were trying to find. The way you worded it was a bit confusing. What mazer is saying is also correct but I assumed it wasn't what you were looking for. Anyway, the easiest and surest way to do this would be to take a gravity reading of your cherry juice before you add it to the must, of course